For example, a simple mechanical float valve might have worked, but it’s not horse friendly and liable to get damaged soon. ... The relay in turn switches the solenoid valve.

Commercial or industrial float valves are too big to use in his small tanks, which led [Billy] to test dozens of DIY designs. ... The relay in turn switches the solenoid valve.

The float, the flapper, or the fill valve are the most likely causes of a toilet running continuously. You can perform easy tests like monitoring water movement to help identify the cause.